One basic technique for the manufacture of powder coating materials is the so-called fusion method. This involves the mixing of solvent-free raw materials in their molten state, usually via some form of extruder, cooling, pulverization and size separation-classification. This method has a number of disadvantages unrelated to pigmentation and an additional short-coming when metal flakes are employed as pigments. The high shear employed in the mixing stage results in deformation of the metal flakes. Additionally, during the pulverization step, the metal flakes are further deformed and reduced in particle size. Coatings produced from such powders are characterized by a low level of brilliance and poor polychromatic appearance.
Another basic technique for the manufacture of powder coating materials is the so-called solution-preparation, solvent-separation technique which can be effected by more than one method. This general technique involves the preparation of a coating material in an organic solvent, separation of the solvent from the paint solids, and size-separation classification. Also, pulverization in some form may or may not be required depending upon the solvent separation method involved.
The separation of the solvent can be carried out by conventional spray drying techniques or by heat exchange separation wherein the components of a paint solution are separated by volatilization of the more volatile solvent and separation of the volatilized solvent from the nonvolatilized paint solids by gravitational forces. Since the metal flakes can be added after pulverization, if pulverization is required when using any of the solvent separation methods, damage to the metal flakes during pulverization can be avoided by using the solution preparation-solvent-separation technique. Problems do arise, however, with respect to distribution and orientation of the metal flakes when the powder coating material is applied to the substrate to be coated. This is particularly true when the method of application is that of electrostatic spray, the method most commonly employed to apply the final coating of paint to automobiles and a variety of other metal manufacturers. In such applications, the flakes tend to orient in a random fashion with a low percentage of the flakes parallel to the substrate. The net result is a high degree of metal protusion with little metallic brilliance and a low gloss factor.
Thus, when either of the aforedescribed methods are used to produce metal-pigmented, powder-paint coatings in accordance with the prior art processes, a substantially higher aluminum to non-metal pigment ratio is required, relative to the same ratio in liquid paints, in order to achieve the same degree of brightness and metallic appearance obtained with liquid paints. Further, the problem of metal flake protrusion remains even when brightness and metallic appearance are achieved.
In liquid paints, it is known to partially coat aluminum flakes used as pigments to increase the electrostatic spray efficiency of such paints. In U.S. Pat. No. 3,575,900, a method is disclosed for precipitating the resin of the solution coating upon the aluminum flake in colloidal form. This solution is then used as such or mixed with another solution for use. The patentee specificially points out that, while it may be convenient to call this encapsulation, it is not intended to denote that the aluminum particles are completely enveloped. The resin disclosed for this purpose is a copolymer of vinyl chloride and monoethylenically unsaturated monomers containing about 60 to about 90 percent by weight vinyl chloride. Aluminum flake is also partially coated in U.S. Pat. No. 3,532,662. Here the coating was carried out with a random copolymer of methyl methacrylate and methacrylic acid adsorbed on the pigment. By this method, a dispersion is made of the solid particles in a liquid continuous phase comprising an organic liquid containing in solution a polymer which is adsorbed by the particles and a stabilizer, and modifying the polarity of the continuous phase so that the polymer is insoluble therein, the stabilizer being a compound containing an anchor component which becomes associated with the adsorbed polymer on the particle surface and a pendant, chain-like component which is solvated by the modified continuous phase and provides a stabilizing sheath around the particles. It is alleged that this improves the "wetting" of the treated particles by the film-forming material dispersion-type coating composition.
Powder paints have certain advantages over conventional liquid paints in that they are essentially free of volatile solvents but they also present problems which differ from the problems encountered with liquid paints. These differences include differences with respect to employment of aluminum flakes as a color producing component. For instance, when flakes partially coated by resin precipitate are employed in liquid paints, there remains the organic solvent and other components of the solution to prevent direct exposure of the flake to the atmosphere and other external influences. Further, in powder paints, if aluminum flake is coated, the coating must be a relatively dry solid and the size, weight and continuity of the organic encapsulation are all factors in affecting the distribution of such particles when electrostatically sprayed with the powder that is the principal film-former of the coating composition.
Coated aluminum flakes, i.e., aluminum flakes individually encapsulated in a continuous thermosettable film, admixed with the particulate principal film-former of a powder paint and electrostatically sprayed on a metal substrate will in a substantial portion orient in parallel relationship to the substrate. This substantially reduces or eliminates flake protrusion. Unfortunately, however, there remains a tendency for these coated to assume a substrate - parallel orientation close to the outer surface of the cured coating. This can produce two undesired results. The first of these is an insufficient appearance of metallic depth in the coating wherein the metal flakes are seen through varying depths of a film which is usually colored with a non-metal color producing component. The second is an undesired "silvery" effect which dominates the non-metal color producing component if the concentration of the near-surface, substrate-parallel flakes is too high.